1. Field of the Invention
The present invention relates to a pressure contact roller for use in a feed mechanism for advancing a sheet of recording paper to a recording position in a printer with roller rotation, and more particularly to a pressure contact roller formed using an elastomer material. The present invention also relates to a printer using such a pressure contact roller.
2. Description of the Related Art
A thermal transfer printer 51 shown in FIG. 9 has been hitherto employed as a recording apparatus in which full-color recording is performed by superimposing thermally sublimating or thermally fusing inks of plural colors successively on a sheet of recording paper.
The thermal transfer printer 51 has a platen 53 in the form of a flat plate disposed at a desired position on a frame 52. In front of the platen 53, a carriage shaft 54 is disposed parallel to the platen 53. A carriage 55 mounting a thermal head 56 thereon is attached to the carriage shaft 54 such that the carriage 55 is able to reciprocate along the carriage shaft 54.
Plural kinds of ribbon cassettes including ink ribbons of different colors set in a coiled state can be mounted on the carriage 55 successively in predetermined order for full-color recording. Behind the platen 53, feed rollers 57 are disposed to be rotatable by a paper feed motor (not shown) to advance a sheet of recording paper to a gap between the platen 53 and the thermal head 56. In the full-color recording, one kind of ribbon cassette is mounted on the carriage 55, and each time one recording cycle using the ink ribbon in the mounted ribbon cassette is completed, the feed roller 57 is rotated so as to return the recording start line (first line) on the sheet of recording paper to the position between the platen 53 and the thermal head 56 in a repeated manner.
Pressure contact rollers 58 are rotatably disposed near the feed rollers 57, and are held in pressure contact with outer circumferential surfaces of the corresponding feed rollers 57 to cooperatively advance the sheet of recording paper while the sheet between the pressure contact rollers 58 and the feed rollers 57. When the feed rollers 57 are rotated to advance the sheet of recording paper, one surface of the sheet of recording paper, on which recording is made, contacts outer circumferential surfaces of the pressure contact rollers 58. In full-color recording, therefore, the outer circumferential surfaces of the pressure contact rollers 58 come into contact with the recording surface several times on which recording has been already made with ink impregnated in the ink ribbon.
Generally, the pressure contact rollers 58 are each manufactured by forming a rod-like roller shaft by resin molding, and then integrally forming a substantially cylindrical roller body made of an elastomer material over an outer circumference of the roller shaft. Because the recording surface of a sheet of recording paper, on which recording is made, contacts the outer circumferential surfaces of the pressure contact rollers 58 as described above, the pressure contact rollers 58 require to be made of such a one of elastomer materials as not affecting the recording surface. More specifically, elastomer materials each usually contain chemicals, e.g., a plasticizer and a coupling agent, for adjustment of hardness. When the pressure contact rollers 58 containing those chemicals come into contact with, as one example of recording paper, a sheet of paper specific for thermal sublimation recording in which an ink dyed layer is formed on the paper surface, the ink dyed layer is swollen due to the chemicals in the pressure contact rollers 58. In the sublimation transfer process of ink, therefore, the amount of ink dyed into the swollen dyed layer is increased and recording at a desired density cannot be achieved.
For the above reason, amide-based thermoplastic elastomers of the plasticizer-free type containing no contaminant components, e.g., a plasticizer and a coupling agent, have been hitherto used as the elastomer materials of the pressure contact rollers 58.
Paper supply rollers 61 are disposed behind the feed rollers 57, and are rotatable by the paper feed motor to supply a sheet of recording paper to the pressure contact position between the feed rollers 57 and the pressure contact rollers 58.
In a rear end portion of the frame 52 behind the paper supply rollers 61, a recording paper holding plate 62 is disposed to hold sheets of recording paper on which recording is to be made by the thermal head 56.
When an image is recorded with the thermal transfer printer 51 including the pressure contact rollers 58 described above, the pressure contact rollers 58 free from contaminant components impose no effects upon the recording paper, and therefore a good full-color image can be obtained.
The conventional pressure contact rollers 58, however, have a problem as follows. Upon the recording surface of a sheet of recording paper, on which recording has been already made, contacting the outer circumferential surfaces of the pressure contact rollers 58, the ink transferred onto the recording surface drifts to the outer circumferential surfaces of the pressure contact rollers. Accordingly, the recording density in areas of the recording surface of the sheet of recording paper from which the ink has drifted becomes lower than that in the remaining area of the recording surface which does not contact the pressure contact rollers.
It is known that the amount of ink drift depends on the amount of hard segment (i.e., the amount of crystal component) contained in the elastomer material of each pressure contact roller. Because the amount of hard segment is a value proportional to the hardness of the elastomer material, it can be said that the amount of ink drift depends on the hardness of the elastomer material. More specifically, FIG. 8 shows data indicating the correlation between material hardness and ink drift resulted from experiments made using three kinds of elastomer materials, i.e., Aramid, Hytrel (polyester-based elastomer) Hytrel is a name of a product made by DuPont-Tray Co., Ltd. Diamid is a name of a product made by Daicel-Huls Ltd. and Diamid (polyamide-based elastomer). As seen from the data of FIG. 8, there is such a tendency that the smaller the material hardness, the greater is the drift of ink to the elastomer material. Note that, in FIG. 8, the ink drift distance, i.e., the distance in the direction of row over which the ink has drifted to the pressure contact roller, is employed as a parameter representing the drift of ink.
Accordingly, one conceivable method for preventing the ink drift is to increase the hardness of the pressure contact roller by coating a urethane- or silicone-based thermosetting surface treatment agent, for example, on the outer circumferential surface of the pressure contact roller. However, if the material hardness is too high, wear or abrasion due to pressure contact with the feed roller 57 is overly increased. A feed roller of projection type having a number of projections formed on the outer circumferential surface in a zigzag pattern has been recently employed to enhance the force of gripping a sheet of recording paper. In the case of employing such a feed roller, particularly, a possibility of causing wear or abrasion of the pressure contact roller is further increased.